1. Field of the Invention
This invention relates in general to subsea wellhead equipment, and in particular to a method and apparatus for batch drilling several wells within a cluster to a selected depth for subsequent completion.
2. Description of the Prior Art
In one subsea drilling technique, a template will be located on the sea floor. The template will have a number of receptacles through which a well will be drilled. Using a floating drilling vessel, the operator may drill each well, cap it, then move to another. At a later date, a platform will be installed over the template. The operator will then tie-back the wells to the platform, run production tubing, and install a surface tree at the platform.
In this technique, a gimbal is mounted to a conductor or conductor housing, which is then lowered onto the receptacle of the template. The gimbal lands on a cone facing upward from the template receptacle. The gimbal allows vertical alignment of the wellhead housing and conductor if the template is oriented other than precisely horizontal, as well as temporary support of the conductor while cement sets.
Normally, the gimbal is mounted to the bottom of a guide base. The operator lowers large diameter conductor pipe into the sea, with an outer or low pressure wellhead housing located at the upper end and with the guide base latched around the outer wellhead housing. The gimbal lands on the cone of the receptacle. The outer wellhead housing or conductor latches into the receptacle. Once the conductor pipe is in place, the well is drilled to a greater depth with a drill bit of a smaller diameter.
Then, the operator will lower a first string of casing through the conductor pipe, with the string having an inner or high pressure wellhead housing located at its upper end. The inner wellhead housing lands in the conductor housing or outer wellhead housing. The operator will cement this string of casing in place.
The operator may then lower a blowout preventer stack onto the inner wellhead housing. The guide base serves to orient the blowout preventer stack. The operator will then drill the well to total depth and install a second string of casing. The operator may then withdraw the guide base and install a corrosion cap to cover and protect the well. Subsequently, the operator will remove the cap once the platform is in place for tieback and completing the well for production. The operator uses guidelines extending from the template and guide base to the drilling vessel for the various operations.
In some cases it is important that no significant weight be transferred to the template. The template may not be structurally strong enough to support all the casing from all the wells, or it may be located on a soft sea bed and may incline or move under weight. Landing the blowout preventer stack and the second string of casing in the inner wellhead housing could transfer substantial weight to the template through the gimbal. The operator may eliminate the gimbal in such a case, either from the beginning or after the first string of casing has been installed. However, this requires the operator to retrieve the guide base and gimbal on a separate trip after installing the first string of casing. He would have to remove the gimbal from the guide base and lower the guide base back in place. This is time consuming in deep water. Alternately, the operator may utilize a larger template, but this is expensive.
While workable, the use of a guide base for installing the inner and outer wellhead housings is cumbersome because of the size of the guide base. Also, removing the gimbal requires an additional trip. Using guidelines is troublesome at times. Systems have been employed utilizing upward facing funnels mounted permanently to the outer wellhead housing rather than using guidelines. These funnels are large and expensive. They can also interfere with certain operations.